Shear wave splitting reveals east-west variations in Subducted Indian plate morphology beneath Southern Tibetan Plateau
Shitan Nie, Xiaobo Tian, Yun Chen, Xiaofeng Liang, Chenglong Wu
Abstract
The subduction of the Indian plate beneath the Tibetan Plateau drives key geodynamic processes, yet east-west variations in slab morphology remain poorly resolved. Seismic anisotropy, measured by teleseismic shear wave splitting (SWS), provides critical insights into mantle deformation linked to subduction geometry. Using a new east-west-oriented broadband seismic array (Tibet-30 N) in the southern Lhasa terrane, we analysed 639 SWS measurements and 527 null results. Our study reveals pronounced lateral variations in fast polarization directions (0°–60°) and splitting times (0–0.9 s). Delayed SKS arrivals (∼2 s) and null splitting near the Yadong–Gulu rift (YGR) suggest asthenospheric upwelling through a slab tear, consistent with low upper-mantle velocities. East of the YGR, large splitting times (0.9 s) and NNE fast directions align with flat subduction, while steep subduction west of the YGR induces vertical mantle flow and reduced anisotropy. Step-like changes in splitting parameters across north‒south-trending rifts imply segmented subduction angles: steep between the Tangra Yum Co and Pumqu–Xianza rifts, transitioning to flat eastward. Integrating previous SWS observations, we propose a model where tearing of the Indian slab beneath rifts facilitates asthenospheric upwelling, while lateral mantle flow reflects slab geometry. Our results highlight east-west variations in Indian plate subduction—flat in eastern and western Tibetan plateau and steep centrally—controlled by slab tearing and mantle dynamics.